1. Field of the Invention
The present invention relates to a measuring device and a measuring method for measuring the wavefront aberration of optical members such as lenses, transparent members, and mirrors.
2. Description of the Related Art
In recent years, high-performance optical systems having low remaining aberration have been required in a variety of fields. For example, in the photolithography process wherein semiconductor devices and the like are produced, a projection lens (projection optical system) having a superior imaging performance, and suppressing wavefront aberration, have been demanded in order to correctly transfer a circuit pattern formed on a reticle or a photo-mask (hereinafter, collectively referred to as reticles) at a predetermined magnification (reduction ratio) to a semiconductor wafer or the like on which a photosensitive material is applied. In particular, recently, to meet the demand for a further miniaturization of semiconductor device, the minimum pattern that a projection lens can transfer, is frequently transferred using the imaging capability of the projection lens to the limit. In a projection lens, therefore, it is necessary to bring the remaining wavefront aberration as nearly zero as possible. For this purpose, endeavors to reduce the remaining aberration are being made from the viewpoints of both optical design and production.
Hitherto, as a measuring device for measuring the wavefront aberration with a high accuracy, interferometers have been frequently used. The light flux used in conventional interferometric sensing has been one in a particular polarized state of light, for example, a linearly polarized light in a certain orientation, or either of a right-handed circularly polarized lights and left-handed circularly polarized lights.
In the inspection of the wavefront aberration of the projection lens for use in a semiconductor exposure system, the performance of the projection lens with respect to incident lights in various polarization states must be grasped very strictly. In this case, due consideration must be given to the influence of the wavefront configuration caused by a slight birefringence existing in a projection lens (lens to be measured). For this purpose, it is necessary to measure the amount of retardation of the lens to be measured caused by birefringence and the average wavefront of the wavefronts separated by the birefringence (hereinafter, referred to as average wavefront). However, in the conventional measuring method, wherein light flux in one polarization state is used, the wavefront of a lens to be measured in which birefringence exists, cannot be measured with a high accuracy.